Your ISP may be performing filtering that does not allow you to route directly to these domains/I.P's. Your broadband connection allows you to get to it from the proxy so either your ISP or even your own Firewall is filtering that domain.
Have you ever tried to access this directly when using a friends, or business connection that does not use your ISP or even one that does and see if you experience the same problem.
Most probably your IP is blocked by the site's firewall. And i presume your IP is static as in case of most of the broadband connections. You can do it manually. Just run the command prompt by typing cmd and then type "ipconfig /release" and type enter. This would disconnect your internet. Then go the Local Area Connection properties in network connections and then right click>properties>TCP/IP>General tab> here you enter a new IP address, doesn't really matter what you enter and then press tab to auto fill the other attributes. >press ok. Once again go to Local Area Connection>properties>check the box "Automatically obtain IP adress">press ok. I hope this would do.